How does router forward packets

In essence, a private address is rewritten to be a public address, often from a pool or range of public addresses supplied by an ISP. This type of NAT is not the most popular because it does not help to dramatically reduce the need for IP addresses—the number of globally routable addresses must equal or exceed the number of internal hosts that wish to access the Internet simultaneously.

A much more popular approach, NAPT involves using the transport-layer identifiers i. This allows a large number of internal hosts i. Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. How does Router know where to forward packet Ask Question. Asked 11 years, 9 months ago. Active 6 years, 11 months ago. Viewed 48k times.

Does this have anything to do with the MAC addresses? How exactly does this work? Improve this question. Kornelije Petak Kornelije Petak 1, 3 3 gold badges 20 20 silver badges 24 24 bronze badges.

All IP packets contain a TTL value that determines the number of router hops that a packet may be routed, this is decremented by most routers although routers can be configured to reduce the value by more than 1 It is used to prevent routing loops, and ensures topologies with loops do not result in packets that circulate indefinitely. At the output interface, the packet together with a new link layer header is placed into a transmit queue until the link layer processor is ready to transmit the packet.

This, like the receive queue. Each out-going packet requires a new link layer protocol header to be added encapsulation with the destination address set to the next system to the receive the packet.

The link protocol controller also maintains the hardware address table associated with the interface. The packet is finally sent using the media interface with the hardware address set to the next hop system.

In this way, transmitted packets have a new link protocol header added prior to transmission of each packet. They can be fragmented by the router into two or more smaller packets. If an IPv4 packet is received which has the Don't Fragment DF bit set in the packet header or uses IPv6, the packet is not fragmented, but is instead discarded. In this case, an ICMP error message is returned to the sender i. The routing and filter tables resemble similar tables in link layer bridges and switches.

Except, that instead of specifying link hardware addresses MAC addresses , the router table sepcify network IP addresses. For more information on the Quagga protocols, refer to the Open Solaris Quagga. For configuration procedures for these protocols, go to the documentation for quagga.

Sites with multiple routers and networks typically administer their network topology as a single routing domain, or autonomous system AS. The following figure shows a typical network topology that would be considered a small AS. This topology is referenced in the examples throughout this section. The figure shows an AS that is divided into three local networks, Four routers share packet-forwarding and routing responsibilities. The AS includes the following types of systems:.

Border routers connect an AS to an external network, such as the Internet. In Figure 5—3 , the border router's interfaces connect to internal network Default routers maintain routing information about all the systems on the local network. In Figure 5—3 , Router 1s interfaces are connected to internal network Router 1 also serves as the default router for Router 1 maintains routing information for all systems on Router 2s interfaces connect to internal network For an example of configuring a default router, refer to Example 5—4.

Packet-forwarding routers forward packets but do not run routing protocols. This type of router receives packets from one of its interfaces that is connected to a single network. These packets are then forwarded through another interface on the router to another local network.

In Figure 5—3 , Router 3 is a packet-forwarding router with connections to networks Multihomed hosts have two or more interfaces that are connected to the same network segment.

A multihomed host can forward packets, which is the default for all systems that run the Solaris OS. Figure 5—3 shows a multihomed host with both interfaces connected to network For an example of configuring a multihomed host, refer to Example 5—6. Single interface hosts rely on the local routers, not only for packet forwarding but also for receiving valuable configuration information. Figure 5—3 includes Host A on the This section contains a procedure and example for configuring an IPv4 router.

Because a router provides the interface between two or more networks, you must assign a unique name and IP address to each of the router's physical network interfaces.

Thus, each router has a host name and an IP address that are associated with its primary network interface, in addition to a minimum of one more unique name and IP address for each additional network interface. You can also use the following procedure to configure a system with only one physical interface by default, a host to be a router. You can configure all interfaces of a router during Solaris system installation. The following instructions assume that you are configuring interfaces for the router after installation.

After the router is physically installed on the network, configure the router to operate in local files mode, as described in How to Configure a Host for Local Files Mode. This configuration ensures that routers boot if the network configuration server is down.

On the system to be configured as a router, assume the Primary Administrator role or become superuser. The Primary Administrator role includes the Primary Administrator profile. The following example output from dladm show-link indicates that a qfe NIC with four interfaces and two bge interfaces are physically available on the system.

The following example output from ifconfig -a shows that the interface qfe0 was configured during installation. This interface is on the The remaining interfaces on the qfe NIC, qfe1 - qfe3 , and the bge interfaces have not been configured.

Interfaces that are explicitly configured with the ifconfig command do not persist across reboots. For example, to assign the IP address The interfaces timbuktu and timbuktu are on the same system. Notice that the network address for timbuktu is different from the network interface for timbuktu. The difference exists because the physical network media for network Network prefixes and their dotted-decimal equivalents can be found in Figure 2—2.

At this point, the router can forward packets beyond the local network. The router also supports static routing , a process where you can manually add routes to the routing table. If you plan to use static routing on this system, then router configuration is complete. However, you need to maintain routes in the system routing table.

For information on adding routes, see Configuring Routes and the route 1M man page. Turn on the default IPv4 routing protocols in either of the following ways:.

For information about the routeadm command, see the routeadm 1M man page. This example shows how to upgrade a system with more than one interface to become a default router. The goal is to make Router 2, which is shown in Figure 5—3 , the default router for network Router 2 contains two wired network connections, one connection to network The example assumes that the router operates in local files mode, as described in How to Configure a Host for Local Files Mode.

After becoming superuser or assuming an equivalent role, you would determine out the status of the system's interfaces. The output of dladm show-link indicates that three links are available on the system. Only the ce0 interface has been plumbed. You would begin default router configuration by physically connecting the bge0 interface to the Then, you would plumb the interface and make it persist across reboots.

Continue by configuring the following network databases with information about the newly plumbed interface and the network to which it is connected:.

All Rights Reserved. The Cisco Learning Network. View This Post. Edited by Admin February 16, at AM. How does a router forward packets inside a LAN once a packet is received? Hi, I am just a little confused, as to how a router would forward a packet inside a LAN once a packet is received on one of its interfaces.

Thanks so much. Amit, The router will check the routing table first.